As a projector, for example, there are known an over-head projector which projects an image of light transmitting through a transparent sheet on which images such as characters and figures are drawn onto a projected surface and a projector which projects an image outputted as a video signal from a computer or video equipment onto a projected surface. The projector has a structure which is arranged apart from a projected surface (screen or the like) and forms an arbitrary image on the projected surface by a projecting optical system, and is also capable of enlarging the arbitrary image and displays it on the projected surface. On the other hand, the projector and the projected surface are separate from each other, so that depending on their relative arrangement relationship, the projected image on the projected surface may be distorted.
As a technique for detecting such distortion of a projected image on a projected surface, for example, the detecting technique disclosed in Patent document 1 is known. In this detecting technique disclosed in Patent document 1, first, a reference brightness image having a light region in a dark region is projected onto a projected surface by a projector. Distortion of the projected image projected onto the projected surface is detected by a photodetecting apparatus that includes a two-dimensional PSD (Position Sensitive Detector) and an imaging optical system. Namely, the photodetecting apparatus forms a projected image on a photosensitive surface of the two-dimensional PSD by the imaging optical system, and detects distortion of the projected image by analyzing an output value from the two-dimensional PSD. A method in which a CCD is used instead of the two-dimensional PSD and distortion of a projected image is detected by performing image operations is also known.    Patent document 1: Japanese Patent Application Laid-Open No. 2004-88169